438
FXUS63 KLOT 052319
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
619 PM CDT Tue Aug 5 2025

.KEY MESSAGES...

- Isolated showers/storms may develop this afternoon, mainly
  near and south of a Gibson City IL to Rensselaer IN line (15%
  chance).

- A gradual increase in heat and humidity will continue through
  the end of the week with peak heat indices of 95-100F Friday
  and Saturday.

&&

.DISCUSSION...
Issued at 248 PM CDT Tue Aug 5 2025

Through Wednesday Night:

Our region remains beneath a baggy mid-level trough aloft, with
relatively weak flow through the column. A mid-level vort was
noted over east-central IN, which continues to drift slowly to
the east. While higher theta-E air mass remains across central
IL/IN as noted by more extensive diurnal cumulus field, there
appears to be little in the way of low-level convergent focus.
Can`t rule out an isolated shower/thunderstorm or two across our
far southeastern cwa later this afternoon, where RAP
mesoanalysis depicts 500+ J/kg of MLCAPE and little/no cap, but
have further trimmed pops to south of about a Gibson City to
Rensselaer line and capped them at 15 percent. Any isolated
storms should dissipate with loss of diurnal heating early this
evening.

Wildfire smoke/haze continues to linger across the Great Lakes
and upper Midwest regions this afternoon, within light east-
southeast synoptic wind field on the southwestern periphery of
high pressure centered over Quebec. HRRR/RAP near-surface smoke
concentration forecasts depict another push into northern IL/IN
tonight into Wednesday morning, and have maintained a mention of
areas of smoke in the forecast for all but our farthest south
tier of counties. In addition, have added a patchy fog mention
toward sunrise Wednesday west and south/southeast of Chicago
proper, as light winds and generally clear skies should again
support radiational cooling sufficient to produce some patchy,
shallow fog.

Similar weather conditions are expected across the area
Wednesday, save for temperatures warming a couple of degrees
more solidly into the mid-80s away from Lake Michigan. Along the
lake, onshore lake breeze will keep readings near 80. Humidity
will creep up slightly as well, though not much as compared to
last week. Other than mainly morning areas of smoke/haze, a
diurnal cu field should provide partly cloudy conditions through
the day. Wednesday night lows will also be a little warmer,
ranging from the mid-60s in outlying areas around 70 in Chicago.

Ratzer


Thursday through Tuesday:

An upper ridge is expected to be building across the central
CONUS heading into Thursday, but a shortwave trough is also
forecast to be pivoting around the northern periphery of the
ridge. Guidance is in fairly good agreement that a thunderstorm
complex should develop with this shortwave in IA and then track
towards northern IL by Thursday morning. While the complex is
forecast to weaken as it nears the Mississippi River due to
lesser instability east of the river, the shortwave is still
expected to pivot over northern IL on Thursday. If sufficient
destabilization can occur as forecast soundings suggest, then
additional showers and thunderstorms may redevelop across parts
of the area Thursday afternoon and evening. Given the weaker
forcing, coverage of any showers and storms will likely be very
hit and miss and thus have decided to maintain the silent 10-14%
POPs offered by the NBM for now. That said, if trends continue
to support showers/storms developing a formal mention may be
needed with future forecast updates.

Any showers/storms that do materialize Thursday
afternoon/evening should gradually taper through the evening
resulting in rain-free conditions for Friday and Saturday as the
aforementioned ridge continues to build into northern IL and
northwest IN. Though, temperatures during this period will
continue to warm with highs forecast to be in the lower 90s
Friday and Saturday. Dew points will also be increasing back
into the upper 60s and lower 70s resulting in heat indices in
the 95-100 degree range during this period. While these values
are below our formal heat advisory criteria, they are still
sufficient to result in heat related illness especially in
sensitive groups so keep that in mind for any outdoor plans
these days.

Heading into Sunday, the ridge will begin to get flattened as a
broad upper trough moves into the northern Plains and Upper
Midwest. While the heat and humidity don`t look to diminish all
that much (highs in the upper 80s into next week), the trough
and any associated shortwaves that traverse along it will bring
the potential for periods of showers and thunderstorms. Given
that there is a good deal of uncertainty as to when and where
any shortwaves will track and thus their respective storm
coverage, have opted to maintain the 20-40% POPs Sunday into
early next week but suspect further refinement to occur with
later forecasts. Regardless, it looks like an active period of
weather may be upon us for next week.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 619 PM CDT Tue Aug 5 2025

East/southeast winds around 10 knots will diminish this evening,
trending light/variable to light north-northwesterly. Winds may
remain light/variable through Wednesday morning, but the
expectation is for winds to pick up a southeasterly direction
through the day. A lake breeze will then push across the GYY,
MDW, and ORD terminals during the afternoon. Some indication
that winds could attempt to flip SSW at ORD during the mid
afternoon, but chances for this remain under 20 percent right
night.

Have introduced 6SM HZ mentions at the Chicago-area terminals
out the gate as upstream observations and webcam imagery reveals
some gradual deterioration in vsbys due to a haze/smoke layer
pushing off the lake. Not seeing any indications vsbys will
meaningfully drop below 6SM, but will keep an eye on trends.

This evening and overnight, BR and/or shallow fog will likely
develop, similar to previous nights, and any additional near-
surface smoke/haze may add to the potential for slowly dropping
vsbys tonight.

Carlaw

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT Wednesday night for
     ILZ005-ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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